As a thermistor, a PTC (Positive Temperature Coefficient) thermistor having positive temperature coefficient of resistance has been known. Since the resistance increases as the temperature rises, the PTC thermistor can be used as a self-controlling type heating element, an over-current protecting element, an overheat detection element or the like. In the prior art, the PTC thermistor has been formed by adding a minute amount of rare earth elements or the like to barium titanate (BaTiO3) which is the main component to be semiconductorized. Therefore, it will have a sharp increase in the resistance by several orders of magnitude above the Curie point, while it has a small resistance under Curie point.
The Curie point of BaTiO3 is usually about 120° C. However, the Curie point can be shifted to a low temperature side by substituting a part of Ba with Sr or Sn. A high Curie point is required particularly for a PTC thermistor used as a heating element, because a PTC thermistor with high Curie point generates high temperature heat source. However, as for the shifting of the Curie point towards a high temperature side, it has been realized by substituting a part of Ba with Pb at present. From the view point of the trend of decreasing the environmental load of the world, practical application of alternative material without Pb has been demanded.
In the following Patent Document 1, a semiconductor ceramic composition without Pb has been disclosed. The composition is produced by preparing BT calcined powder consisiting of (BaR)TiO3 (where R is at least one rare earth element) calcined powder or Ba(TiM)O3 (where M is at least one of Nb and Sb) calcined powder, and BNT calcined powder consisiting of (BiNa)TiO3 calcined powder, respectively, sintering the molded body prepared from the mixed calcined powders of the BT calcined powder and the BNT calcined powder in an atmosphere containing 1 vol % or less of oxygen, and then subjecting the sintered body to a heat-treatment for 0.5 hours or more and 24 hours or less in a temperature of 300° C. or more and 600° C. or less in an atmosphere containing 0.1 vol % or more of hydrogen.
According to the following Patent Document 1, it has been described that a semiconductor ceramic composition without using Pb, which has a Curie point shifted to a high temperature side higher than 120° C., a small resistivity at room temperature, and a lager temperature coefficient of resistance α, can be obtained.